In the 3rd Generation Partnership Project, 3GPP, ongoing specification work is performed for the establishment of operational performances for inter-work between Evolved Universal Terrestrial Radio Access Network, E-UTRAN, and GSM EDGE Radio Access Network, GERAN, and UTRAN. E-UTRAN is also known as Long Term Evolution, LTE, Radio Access Network, RAN, and is the RAN part for the System Architecture Evolution, SAE, Core Network, CN. The latest name for SAE is Enhanced Packet Core, EPC. Here, “Inter-work”, as mentioned above, means that a User Equipment, UE, with active packet sessions is moving from LTE to GERAN coverage or in the other direction and the services using the active packet sessions are effectively moved to the target RAN and cell.
The current assumption is that both Packet Switched Handover (hereinafter called PS HO or PS handover), and enhanced Network Assisted Cell Change, eNACC, will be standardized for the purpose of moving between GERAN/UTRAN and LTE. It is also likely that PS HO can be used in both directions and that the eNACC only applies for the case when the UE is moving from LTE to GERAN. Both PS HO and eNACC are partly based on the same mechanisms.
In addition, work is ongoing in 3GPP to allow for the possibility to perform so called Inter-domain transfer from PS domain to CS domain. This domain transfer is applicable, for example, for the case when:                i) a UE is using the packet bearers in LTE for Real-time services,        ii) the UE is moving from LTE to GERAN coverage and        iii) the target GERAN RAN/cell does not support real-time services in the PS domain.        
One example is the handover of E-UTRAN Voice over Internet Protocol (IP), VoIP, call to CS voice resources in GERAN. This type of inter-RAT (Radio Access Technology) handover also requires PS to CS domain transfer, i.e. inter domain transfer.
There exists today a proposal (GP-070675) on how to perform the above mentioned Inter-domain transfer from E-UTRAN PS domain to GERAN CS domain. This proposal is currently getting more and more support in the standardization of GERAN. One main principle is that the target RAT/cell decides whether Inter-Domain change needs to be performed. If the target RAT/cell decides to perform this Inter-Domain change, then both PS and CS radio resources are allocated for the UE during the packet switched handover preparation phase, after the target BSS has received the PS Handover Required message. The allocated resources are included in a new message (e.g. Inter-Domain Handover Command) that in turn is included in the Target BSS to Source BSS Transparent Container which is included in the PS Handover Request Acknowledge message.
When the related steps in the PS Handover preparation phase signaling are performed, the UE is commanded to perform combined PS/CS Handover (or this could be also called Inter Domain, ID, Handover) to the target GERAN cell.
The next step is then that the UE would need to perform Location Area Update, LAU, towards the MSC controlling the target GERAN cell. Please note that this step could be optional if the UE is already CS Attached in the MSC. This could be the case for example because of some CSoLTE MM procedures performed via LTE. If Gs-interface is supported in the core network CN controlling the target GERAN cell, then a Combined Routing Area Update and Location Area update could be possible. Otherwise, the UE would need to perform normal location area update towards the MSC.
The next step after successful Location Area update, LAU, is that a Voice Call Continuity, VCC, application in the MS would need to make a CS call towards the VCC Application Server, AS, in the IMS CN. Once this call establishment is successfully performed, the VCC AS may route the call leg via the CS domain and finalize the Inter-Domain change.
The Location Update procedure depends on Network operation mode. When the target network is in Network Operation Mode I, i.e. the Gs interface between the SGSN and the MSC is supported, the terminal performs Combined Routing Area (RA) and Location Area (LA) Update procedures. However, when the target network is in Network Operation Mode II or III (i.e. the Gs interface between the SGSN and the MSC is not supported). In this case, the terminal performs separate Location Update and Routing Area Update procedures.
The problem with the existing solution is that the proposed sequence for the different scenarios takes time because of interruptions related to the Inter-Domain change.